Basically, radar systems operate by transmitting an electromagnetic waveform from an antenna. The waveform can then be reflected by an object (target) as an echo (return signal). When this echo (return signal) is received, it is processed for presentation on a radar indicator. In a perfect case, one wherein there isn't any unwanted interference, the radar return signal (echo) can be processed to obtain clearly observable indications of target range, azimuth angle (θ), and elevation/depression angle (γ), relative to the radar antenna. In an actual real-time application, however, there will always be clutter and interference.
In order to provide for more robust systems, having clearer radar indications with accurate signal resolution and enhanced performance characteristics, several improved radar architectures have been proposed. For instance, one such radar system architecture may include Multiple Inputs [i.e. transmitters] and Multiple Outputs [i.e. receivers] (MIMO). For each of these architectures the objective has been to reduce clutter and increase azimuth estimation by increasing the system's space-time Degrees of Freedom (DoFs). This is a particularly important consideration for radar systems having relatively small apertures.
As used in this disclosure, each DoF is considered as being a separate perspective of the target. Technically, a DoF can be established geometrically (e.g. based on transmitter or receiver location), or electronically (e.g. phase shift in the transmitter radar beam). In either case, each DoF is established using a unique transmitter and a unique receiver. An important aspect of this is that for a given target, the more DoFs (perspectives) there are, the more pertinent information there is for signal processing purposes. For example, a radar system having two transmitters and two receivers will have four DoFs, and will have more useable information than can be generated with fewer DoFs. The particular application, however, may limit the number of transmitters and receivers that can be effectively used in a system.
In addition to the useable DoFs a system may have, there are other important operational considerations. For the particular case of an Unmanned Aerial Vehicle (UAV), the flight envelope of the UAV is a major consideration. Of particular interest here is an ability for a radar system that is mounted on a UAV to function properly both while the UAV is stationary, and while it is in forward flight. More particularly, for a well known Ground Moving Target Indications (GMTI) mode of operation, it is best if the radar system is held stationary and uses a narrow band waveform. On the other hand, for a Synthetic Aperture Radar (SAR) mode of operation, it is best if the radar system is moving and employs a much wider bandwidth. For a UAV application, both modes of operation are needed.
In light of the above, it is an object of the present invention to provide a radar antenna system that uses an innovative wideband waveform concept for simultaneous SAR and GMTI operations. Another object of the present invention is to provide a radar antenna system that will enable small UAV radars to detect, track and image both vehicles and dismounts in a wide range of operational environments. Yet another object of the present invention is to provide a radar antenna system that uses a single transmitter with a wideband waveform to alternately transmit pulses with different phases (2 DoFs), and that uses two receivers (2 DoFs) to simultaneously receive echoes having the different phases, to establish a system having four DoFs. Still another object of the present invention is to provide a radar antenna system that is easy to use, is relatively simple to manufacture and is comparatively cost effective.